This invention relates generally to quartz pressure transducers and more particularly to quartz pressure transducers having thickness shear mode quartz crystal resonators.
Pressure, such as in an oil or gas well, for example, can be measured in a known manner using a quartz pressure transducer. Such a transducer includes a quartz crystal resonator which is a piezoelectric element that changes an electrical characteristic of an electrical circuit in response to mechanical stress induced in the resonator by the pressure to be measured. Typically, the resonator is part of an oscillator circuit that generates a sinusoidal electrical signal having a frequency which varies with the response of the resonator.
One type of pressure transducer, particularly suited for sensing pressure in an oil or gas well, has a thickness shear mode quartz crystal resonator. In a thickness shear mode quartz crystal resonator, electrodes are affixed to the two major surfaces of the resonator and the electrical response arises from stress across the thickness of the resonator, which thickness is perpendicular to the major surfaces. Although others have been proposed, two particular thickness shear mode quartz crystal resonators extensively, if not exclusively, used include either AT-cut quartz crystal or BT-cut quartz crystal.
AT-cut and BT-cut quartz crystal resonators are useful at least in part because their response to pressure is substantially independent of temperature. That is, these resonators are said to have a zero temperature coefficient of frequency. Both of these particular resonators are, however, limited as to the maximum pressure and temperature at which they can be used. This limitation is reached when the stress on the resonator reaches a maximum level at or beyond which the quartz crystal either fractures or "twins".
For a BT-cut resonator, the primary failure is fracturing. This can occur such as at approximately 12,000 pounds per square inch (psi) at about 175 degrees centigrade (.degree.C.) for one particular embodiment of the BT-cut pressure sensor. For an AT-cut resonator, the primary failure is "twinning". For one particular embodiment, this can occur such as at approximately 20,000 psi at about 175.degree. C. or approximately 16,000 psi at about 200.degree. C. or some other comparable combination of pressure and temperature which would result in the maximum allowable stress level in the resonator. For another embodiment of the AT-cut sensor, this can occur at approximately 20,000 psi at about 200.degree. C. or approximately 25,000 psi at about 175.degree. C. or some comparable combination of pressure and temperature.
When quartz "twins" due to excessive stress, the crystalline structure is suddenly altered to a more stable state. For AT-cut crystal, an electrical twin of this type can be regarded as a 180.degree. rotation about the Z-axis of the X, Y and Z crystallographic axes relative to which the crystal can be oriented in known manner. Theoretically, a twin could be reversed by applying to the resonator a stress pattern that would make the original state the lower energy state for the stress pattern applied. This is not presently practical; therefore, once a crystal is twinned it remains in that state. The pressure response of the AT-cut crystal in the twinned state is more sensitive to temperature than in its original, untwinned state. Furthermore, when a crystal twins, it typically does so only partially. Such a partially or imperfectly twinned crystal cannot be made to resonate so that it is not useful as a resonator of the types referred to herein.
Because of the maximum pressure and temperature limitations of the widely used AT-cut and BT-cut resonators, there is the need for an improved pressure transducer having a quartz crystal of such a type as to enable its use in place of AT-cut or BT-cut crystal throughout the normal operating range of AT-cut and BT-cut crystals as well as at higher pressures and temperatures and yet provide a suitable pressure response.